The kinetic phase behavior and phase transformation paths of purified tristearoylglycerol (SSS), 3-palmitoyl-1,2-distearoyl-sn-glycerol (PSS) and 1,2-dipalmitoyl-3-stearoyl-sn-glycerol (PPS) were investigated in terms of polymorphism, crystallization and melting. The details of the phase transformation paths were obtained using the heating cycles of two sets of experiments: (a) cooling rate was varied and heating rate fixed and (b) cooling rate was fixed and heating rate varied. Kinetic effects were manifest in all measured properties, underscoring the complexity of the phase transformation paths for each TAG, and the intricate thermodynamics-molecular relationships. For the first time, XRD data obtained for SSS. PSS and PPS TAGs, cooled at rates higher than 0.5 degrees C/min, suggested the formation of a transient structure similar to the so-called alpha(2)-phase which has been observed in mixed saturated-unsaturated TAGs quenched from the melt. The more stable phases (beta' in PSS and PPS, and beta in SSS) were only observed for cooling rates lower than 1.0 degrees C/min. The kinetic and thermodynamic differences observed in the crystallization, structure and melting of SSS, PSS and PPS are proposed to be mainly due to the disturbances introduced at the "terrace" level via methyl-end group interactions, i.e., the missing of two or four CH2 groups compared to SSS. The symmetrical SSS with a relatively flat "terrace" crystallizes preferably in the most stable beta-form. Two missing CH2 groups at the sn-1 position (PSS) introduces enough structural disturbances to promote the relative prevalence and persistence of the beta'-phase, and four missing CH2 groups at the sn-1 and sn-2 positions (PPS) is relatively too large of a disturbance and therefore favors the alpha-form. (C) 2011 Elsevier Ireland Ltd. All rights reserved.
